A. The SELEX Process
The dogma for many years was that nucleic acids had primarily an informational role. Through a method known as Systematic Evolution of Ligands by Exponential enrichment, termed SELEX, it has become clear that nucleic acids have three dimensional structural diversity not unlike proteins. SELEX is a method for the in vitro evolution of nucleic acid molecules with highly specific binding to target molecules and is described in U.S. patent application Ser. No. 07/536,428, filed Jun. 11, 1990, entitled “Systematic Evolution of Ligands by Exponential Enrichment,” now abandoned, U.S. patent application Ser. No. 07/714,131, filed Jun. 10, 1991, entitled “Nucleic Acid Ligands,” now U.S. Pat. No. 5,475,096, and U.S. patent application Ser. No. 07/931,473, filed Aug. 17, 1992, entitled “Methods for Identifying Nucleic Acid Ligands,” now U.S. Pat. No. 5,270,163 (see also WO 91/19813), each of which is specifically incorporated by reference herein. Each of these applications, collectively referred to herein as the SELEX patent applications, describes a fundamentally novel method for making a Nucleic Acid Ligand to any desired target molecule. The SELEX process provides a class of products which are referred to as Nucleic Acid Ligands, each ligand having a unique sequence, and which has the property of binding specifically to a desired target compound or molecule. Each SELEX-identified Nucleic Acid Ligand is a specific ligand of a given target compound or molecule. SELEX is based on the unique insight that Nucleic Acids have sufficient capacity for forming a variety of two- and three-dimensional structures and sufficient chemical versatility available within their monomers to act as ligands (form specific binding pairs) with virtually any chemical compound, whether monomeric or polymeric. Molecules of any size or composition can serve as targets.
The SELEX method involves selection from a mixture of candidate oligonucleotides and step-wise iterations of binding, partitioning and amplification, using the same general selection scheme, to achieve virtually any desired criterion of binding affinity and selectivity. Starting from a mixture of Nucleic Acids, preferably comprising a segment of randomized sequence, the SELEX method includes steps of contacting the mixture with the target under conditions favorable for binding, partitioning unbound Nucleic Acids from those Nucleic Acids which have bound specifically to target molecules, dissociating the Nucleic Acid-target complexes, amplifying the Nucleic Acids dissociated from the Nucleic Acid-target complexes to yield a ligand-enriched mixture of Nucleic Acids, then reiterating the steps of binding, partitioning, dissociating and amplifying through as many cycles as desired to yield highly specific high affinity Nucleic Acid Ligands to the target molecule.
It has been recognized by the present inventors that the SELEX method demonstrates that Nucleic Acids as chemical compounds can form a wide array of shapes, sizes and configurations, and are capable of a far broader repertoire of binding and other functions than those displayed by Nucleic Acids in biological systems.
The present inventors have recognized that SELEX or SELEX-like processes could be used to identify Nucleic Acids which can facilitate any chosen reaction in a manner similar to that in which Nucleic Acid Ligands can be identified for any given target. In theory, within a Candidate Mixture of approximately 1013 to 1018 Nucleic Acids, the present inventors postulate that at least one Nucleic Acid exists with the appropriate shape to facilitate each of a broad variety of physical and chemical interactions.
The basic SELEX method has been modified to achieve a number of specific objectives. For example, U.S. patent application Ser. No. 07/960,093, filed Oct. 14, 1992, entitled “Method for Selecting Nucleic Acids on the Basis of Structure,” now abandoned, describes the use of SELEX in conjunction with gel electrophoresis to select Nucleic Acid molecules with specific structural characteristics, such as bent DNA. U.S. patent application Ser. No. 08/123,935, filed Sep. 17, 1993, entitled “Photoselection of Nucleic Acid Ligands,” describes a SELEX based method for selecting Nucleic Acid Ligands containing photoreactive groups capable of binding and/or photocrosslinking to and/or photoinactivating a target molecule. U.S. patent application Ser. No. 08/134,028, filed Oct. 7, 1993, entitled “High-Affinity Nucleic Acid Ligands That Discriminate Between Theophylline and Caffeine,” now U.S. Pat. No. 5,580,737, describes a method for identifying highly specific Nucleic Acid Ligands able to discriminate between closely related molecules, which can be non-peptidic, termed Counter-SELEX. U.S. patent application Ser. No. 08/143,564, filed Oct. 25, 1993, entitled “Systematic Evolution of Ligands by Exponential Enrichment: Solution SELEX,” now U.S. Pat. No. 5,567,588, describes a SELEX-based method which achieves highly efficient partitioning between oligonucleotides having high and low affinity for a target molecule.
The SELEX method encompasses the identification of high-affinity Nucleic Acid Ligands containing modified nucleotides conferring improved characteristics on the ligand, such as improved in vivo stability or improved delivery characteristics. Examples of such modifications include chemical substitutions at the ribose and/or phosphate and/or base positions. SELEX-identified Nucleic Acid Ligands containing modified nucleotides are described in U.S. patent application Ser. No. 08/117,991, filed Sep. 8, 1993, entitled “High Affinity Nucleic Acid Ligands Containing Modified Nucleotides,” now U.S. Pat. No. 5,660,985, that describes oligonucleotides containing nucleotide derivatives chemically modified at the 5- and 2′-positions of pyrimidines. U.S. patent application Ser. No. 08/134,028, supra, describes highly specific Nucleic Acid Ligands containing one or more nucleotides modified with 2′-amino (2′-NH2), 2′-fluoro (2′-F), and/or 2′-O-methyl (2′-OMe). U.S. patent application Ser. No. 08/264,029, filed Jun. 22, 1994, entitled “Novel Method of Preparation of Known and Novel 2′ Modified Nucleosides by Intramolecular Nucleophilic Displacement,” now abandoned, describes oligonucleotides containing various 2′-modified pyrimidines.
The SELEX method encompasses combining selected oligonucleotides with other selected oligonucleotides and non-oligonucleotide functional units as described in U.S. patent application Ser. No. 08/284,063, filed Aug. 2, 1994, entitled “Systematic Evolution of Ligands by Exponential Enrichment: Chimeric SELEX,” now U.S. Pat. No. 5,637,459, and U.S. patent application Ser. No. 08/234,997, filed Apr. 28, 1994, entitled “Systematic Evolution of Ligands by Exponential Enrichment: Blended SELEX,” now U.S. Pat. No. 5,683,867, respectively. These applications allow the combination of the broad array of shapes and other properties, and the efficient amplification and replication properties, of oligonucleotides with the desirable properties of other molecules.
The SELEX method further encompasses combining selected nucleic acid ligands with lipophilic compounds or non-immunogenic, high molecular weight compounds in a diagnostic or therapeutic complex as described in U.S. patent application Ser. No. 08/434,465, filed May 4, 1995, entitled Nucleic Acid Ligand Complexes,” now U.S. Pat. No. 6,011,020. VEGF Nucleic Acid Ligands that are associated with a Lipophilic Compound, such as diacyl glycerol or dialkyl glycerol, in a diagnostic or therapeutic complex are described in U.S. patent application Ser. No. 08/739,109, filed Oct. 25, 1996, entitled “Vascular Endothelial Growth Factor (VEGF) Nucleic Acid Ligand Complexes,” now U.S. Pat. No. 5,859,228. VEGF Nucleic Acid Ligands that are associated with a Lipophilic Compound, such as a glycerol lipid, or a Non-Immunogenic, High Molecular Weight Compound, such as polyethylene glycol, are further described in U.S. patent application Ser. No. 08/897,351, filed Jul. 21, 1997, entitled “Vascular Endothelial Growth Factor (VEGF) Nucleic Acid Ligand Complexes,” now U.S. Pat. No. 6,051,698. VEGF Nucleic Acid Ligands that are associated with a non-immunogenic, high molecular weight compound or lipophilic compound are also further described in PCT/US97/18944, filed Oct. 17, 1997 (WO 98/18480), entitled “Vascular Endothelial Growth Factor (VEGF) Nucleic Acid Ligand Complexes.” Each of the above described patent applications which describe modifications of the basic SELEX procedure are specifically incorporated by reference herein in their entirety.
B. Lipid Constructs
Lipid Bilayer Vesicles are closed, fluid-filled microscopic spheres which are formed principally from individual molecules having polar (hydrophilic) and non-polar (lipophilic) portions. The hydrophilic portions may comprise phosphato, glycerylphosphato, carboxy, sulfato, amino, hydroxy, choline or other polar groups. Examples of lipophilic groups are saturated or unsaturated hydrocarbons such as alkyl, alkenyl or other lipid groups. Sterols (e.g., cholesterol) and other pharmaceutically acceptable adjuvants (including anti-oxidants like alpha-tocopherol) may also be included to improve vesicle stability or confer other desirable characteristics.
Liposomes are a subset of these bilayer vesicles and are comprised principally of phospholipid molecules that contain two hydrophobic tails consisting of fatty acid chains. Upon exposure to water, these molecules spontaneously align to form spherical, bilayer membranes with the lipophilic ends of the molecules in each layer associated in the center of the membrane and the opposing polar ends forming the respective inner and outer surface of the bilayer membrane(s). Thus, each side of the membrane presents a hydrophilic surface while the interior of the membrane comprises a lipophilic medium. These membranes may be arranged in a series of concentric, spherical membranes separated by thin strata of water, in a manner not dissimilar to the layers of an onion, around an internal aqueous space. These multilamellar vesicles (MLV) can be converted into small or Unilamellar Vesicles (UV), with the application of a shearing force.
The therapeutic use of liposomes includes the delivery of drugs which are normally toxic in the free form. In the liposomal form, the toxic drug is occluded, and may be directed away from the tissues sensitive to the drug and targeted to selected areas. Liposomes can also be used therapeutically to release drugs over a prolonged period of time, reducing the frequency of administration. In addition, liposomes can provide a method for forming aqueous dispersions of hydrophobic or amphiphilic drugs, which are normally unsuitable for intravenous delivery.
In order for many drugs and imaging agents to have therapeutic or diagnostic potential, it is necessary for them to be delivered to the proper location in the body, and the liposome can thus be readily injected and form the basis for sustained release and drug delivery to specific cell types, or parts of the body. Several techniques can be employed to use liposomes to target encapsulated drugs to selected host tissues, and away from sensitive tissues. These techniques include manipulating the size of the liposomes, their net surface charge, and their route of administration. MLVs, primarily because they are relatively large, are usually rapidly taken up by the reticuloendothelial system (principally the liver and spleen). UVs, on the other hand, have been found to exhibit increased circulation times, decreased clearance rates and greater biodistribution relative to MLVs.
Passive delivery of liposomes involves the use of various routes of administration, e.g., intravenous, subcutaneous, intramuscular and topical. Each route produces differences in localization of the liposomes. Two common methods used to direct liposomes actively to selected target areas involve attachment of either antibodies or specific receptor ligands to the surface of the liposomes. Antibodies are known to have a high specificity for their corresponding antigen and have been attached to the surface of liposomes, but the results have been less than successful in many instances. Some efforts, however, have been successful in targeting liposomes to tumors without the use of antibodies, see, for example, U.S. Pat. No. 5,019,369, U.S. Pat. No. 5,441,745, or U.S. Pat. No. 5,435,989.
An area of development aggressively pursued by researchers is the delivery of agents not only to a specific cell type but into the cell's cytoplasm and, further yet, into the nucleus. This is particularly important for the delivery of biological agents such as DNA, RNA, ribozymes and proteins. A promising therapeutic pursuit in this area involves the use of antisense DNA and RNA oligonucleotides for the treatment of disease. However, one major problem encountered in the effective application of antisense technology is that oligonucleotides in their phosphodiester form are quickly degraded in body fluids and by intracellular and extracellular enzymes, such as endonucleases and exonucleases, before the target cell is reached. Intravenous administration also results in rapid clearance from the bloodstream by the kidney, and uptake is insufficient to produce an effective intracellular drug concentration. Liposome encapsulation protects the oligonucleotides from the degradative enzymes, increases the circulation half-life and increases uptake efficiency as a result of phagocytosis of the Liposomes. In this way, oligonucleotides are able to reach their desired target and to be delivered to cells in vivo.
A few instances have been reported where researchers have attached antisense oligonucleotides to Lipophilic Compounds or Non-Immunogenic, High Molecular Weight Compounds. Antisense oligonucleotides, however, are only effective as intracellular agents. Antisense oligodeoxyribonucleotides targeted to the epidermal growth factor (EGF) receptor have been encapsulated into Liposomes linked to folate via a polyethylene glycol spacer (folate-PEG-Liposomes) and delivered into cultured KB cells via folate receptor-mediated endocytosis (Wang et al. (1995) Proc. Natl. Acad. Sci. USA 92:3318-3322). In addition, alkylene diols have been attached to oligonucleotides (Weiss et al., U.S. Pat. No. 5,245,022). Furthermore, a Lipophilic Compound covalently attached to an antisense oligonucleotide has been demonstrated in the literature (EP 462 145 B1).
Loading of biological agents into liposomes can be accomplished by inclusion in the lipid formulation or loading into preformed liposomes. Passive anchoring of oligopeptide and oligosaccharide ligands to the external surface of liposomes has been described (Zalipsky et al. (1997) Bioconjug. Chem. 8:111-118).
C. PDGF
Platelet-derived growth factor (PDGF) was originally isolated from platelet lysates and identified as the major growth-promoting activity present in serum but not in plasma. Two homologous PDGF isoforms have been identified, PDGF A and B, which are encoded by separate genes (on chromosomes 7 and 22). The most abundant species from platelets is the AB heterodimer, although all three possible dimers (AA, AB and BB) occur naturally. Following translation, PDGF dimers are processed into ˜30 kDa secreted proteins. Two cell surface proteins that bind PDGF with high affinity have been identified, α and β (Heldin et al. (1981) Proc. Natl. Acad. Sci., 78: 3664; Williams et al. (1981) Proc. Natl. Acad. Sci., 79: 5867). Both species contain five immunoglobulin-like extracellular domains, a single transmembrane domain and an intracellular tyrosine kinase domain separated by a kinase insert domain. The functional high affinity receptor is a dimer and engagement of the extracellular domain of the receptor by PDGF results in cross-phosphorylation (one receptor tyrosine kinase phosphorylates the other in the dimer) of several tyrosine residues. Receptor phosphorylation leads to a cascade of events that results in the transduction of the mitogenic or chemotactic signal to the nucleus. For example, in the intracellular domain of the PDGF β receptor, nine tyrosine residues have been identified that when phosphorylated interact with different src-homology 2 (SH2) domain-containing proteins including phospholipase C-g, phosphatidylinositol 3′-kinase, GTPase-activating protein and several adapter molecules like Shc, Grb2 and Nck (Heldin (1995) Cell 80: 213). In the last several years, the specificities of the three PDGF isoforms for the three receptor dimers (αα, αβ, and ββ) has been elucidated. The α-receptor homodimer binds all three PDGF isoforms with high affinity, the β-receptor homodimer binds only PDGF BB with high affinity and PDGF AB with approximately 10-fold lower affinity, and the αβ-receptor heterodimer binds PDGF BB and PDGF AB with high affinity (Westermark & Heldin (1993) Acta Oncologica 32:101). The specificity pattern results from the ability of the A-chain to bind only to the α-receptor and of the B-chain to bind to both a and β-receptor subunits with high affinity.
The role of PDGF in proliferative diseases, such as cancer, restenosis, fibrosis, angiogenesis, and wound healing has been established.
PDGF in Cancer
The earliest indication that PDGF expression is linked to malignant transformation came with the finding that the amino acid sequence of PDGF-B chain is virtually identical to that of p28sis, the transforming protein of the simian sarcoma virus (SSV) (Waterfield et al. (1983) Nature 304:35; Johnsson et al. (1984) EMBO J. 3:921). The transforming potential of the PDGF-B chain gene and, to a lesser extent, the PDGF-A gene was demonstrated soon thereafter (Clarke et al. (1984) Nature 308:464; Gazit et al. (1984) Cell 39:89; Beckmann et al. Science 241:1346; Bywater et al. (1988) Mol. Cell. Biol. 8:2753). Many tumor cell lines have since been shown to produce and secrete PDGF, some of which also express PDGF receptors (Raines et al. (1990) Peptide Growth Factors and Their Receptors, Springer-Verlag, Part I, p 173). Paracrine and, in some cell lines, autocrine growth stimulation by PDGF is therefore possible. For example, analysis of biopsies from human gliomas has revealed the existence of two autocrine loops: PDGF-B/β-receptor in tumor-associated endothelial cells and PDGF-A/α-receptor in tumor cells (Hermansson et al. (1988) Proc. Natl. Acad. Sci. 85:7748; Hermansson et al. (1992) Cancer Res. 52:3213). The progression to high grade glioma was accompanied by the increase in expression of PDGF-B and the β-receptor in tumor-associated endothelial cells and PDGF-A in glioma cells. PDGF overexpression may thus promote tumor growth either by directly stimulating tumor cells or by stimulating tumor-associated stromal cells (e.g., endothelial cells). The proliferation of endothelial cells is a hallmark of angiogenesis. Increased expression of PDGF and/or PDGF receptors has also been observed in other malignancies including fibrosarcoma (Smits et al. (1992) Am. J. Pathol. 140:639) and thyroid carcinoma (Heldin et al. (1991) Endocrinology 129:2187).
PDGF in Cardiovascular Disease
Percutaneous transluminal coronary angioplasty (PTCA) has become the most common treatment for occlusive coronary artery disease (CAD) involving one or two coronary arteries. In the United States alone about 500,000 procedures are being done annually, with projections of over 700,000 procedures by the year 2000 and about double those amounts worldwide. PTCA, while it involves manipulations inside of coronary arteries, is not considered to be a cardiac surgical intervention. During the most common PTCA procedure, a balloon catheter is threaded through a femoral artery and is positioned within the plaque-laden segment of an occluded coronary vessel; once in place, the balloon is expanded at high pressure, compressing the plaque and increasing the vessel lumen. Unfortunately, in 30-50% of PTCA procedures, reocclusion gradually develops over a period of several weeks or months due to cellular events in the affected vessel wall. Once reocclusion achieves 50% or greater reduction of the original vessel lumen, clinical restenosis is established in the vessel.
In view of the increasing popularity of coronary angioplasty as a less invasive alternative to bypass surgery, restenosis is a serious medical problem. Smooth muscle cells (SMCs) represent a major component of the restenosis lesions. In uninjured arteries, SMCs reside primarily in the medial vessel layer (tunical media). Upon balloon injury that removes the endothelial cells from the intimal layer (tunical intima), SMCs proliferate and migrate into the intima, forming neointimal thickening characteristic of restenosis lesions. When restenosis occurs subsequent to angioplasty, it is usually treated by repeat angioplasty, with or without placement of a stent, or by vascular graft surgery (bypass).
A stent is a rigid cylindrical mesh that, once placed and expanded within a diseased vessel segment, mechanically retains the expanded vessel wall. The stent is deployed by catheter and, having been positioned at the desired site, is expanded in situ by inflation of a high pressure balloon. Being rigid and non-compressible, the expanded stent achieves and maintains a vessel lumen diameter comparable to that of adjacent non-diseased vessel; being pressed tightly into the overlying intima/media, it is resistant to migration within the vessel in response to blood flow. PTCA with stent placement has been compared with PTCA alone and shown to reduce restenosis to about half and to significantly improve other clinical outcomes such as myocardial infarction (MI) and need for bypass surgery.
There is now considerable evidence that PDGF B-chain is a major contributor to the formation of neointimal lesions. In a rat model of restenosis, the neointimal thickening was inhibited with anti-PDGF-B antibodies (Ferns (1991) Science 253:1129-1132; Rutherford et al. (1997) Atherosclerosis 130:45-51). Conversely, the exogenous administration of PDGF-BB promotes SMC migration and causes an increase in neointimal thickening (Jawien et al. (1992) J. Clin. Invest. 89:507-511). The effect of PDGF-B on SMCs is mediated through PDGF β-receptor which is expressed at high levels in these cells after balloon injury (Lindner and Reidy (1995) Circulation Res. 76:951-957). Furthermore, the degree of neointimal thickening following balloon injury was found to be inversely related to the level of expression of PDGF β-receptor at the site of injury (Sirois et al. (1997) Circulation 95:669-676).
U.S. Pat. No. 5,171,217 discloses a method and composition for delivery of a drug to an affected intramural site for sustained release in conjunction with or following balloon catheter procedures, such as angioplasty. The drug may be selected from a variety of drugs known to inhibit smooth muscle cell proliferation, including growth factor receptor antagonists for PDGF.
U.S. Pat. No. 5,593,974 discloses methods for treating vascular disorders, such as vascular restenosis, with antisense oligonucleotides. The method is based on localized application of the antisense oligonucleotides to a specific site in vivo. The oligonucleotides can be applied directly to the target tissue in a mixture with an implant or gel, or by direct injection or infusion.
U.S. Pat. No. 5,562,922 discloses a method for preparing a system suitable for localized delivery of biologically active compounds to a subject. The method relates to treating polyurethane coated substrate with a coating expansion solution under conditions that will allow penetration of the biologically active compound throughout the polyurethane coating. Substrates suitable for this invention include, inter alia, metallic stents. Biologically active compounds suitable for use in this invention include, inter alia, lipid-modified oligonucleotides.
Rutherford et al. (1997, Atherosclerosis 130:45-51) report substantial inhibition of neointimal response to balloon injury in rat carotid artery using a combination of antibodies to PDGF-BB and basic fibroblast growth factor (bFGF).
PDGF in Renal Disease
A large variety of progressive renal diseases are characterized by glomerular mesangial cell proliferation and matrix accumulation (Slomowitz et al. (1988) New Eng. J. Med. 319:1547-1548) which leads to fibrosis. PDGF B-chain appears to have a central role in driving both of these processes given that 1) mesangial cells produce PDGF in vitro and various growth factors induce mesangial proliferation via induction of auto- or paracrine PDGF B-chain synthesis; 2) PDGF B-chain and its receptor are overexpressed in many glomerular diseases; 3) infusion of PDGF-BB or glomerular transfection with a PDGF B-chain cDNA can induce selective mesangial cell proliferation and matrix accumulation in vivo; and 4) PDGF B-chain or β-receptor knock-out mice fail to develop a mesangium (reviewed in Floege and Johnson (1995) Miner. Electrolyte Metab. 21:271-282). In addition to contributing to kidney fibrosis, PDGF is also believed to play a role in fibrosis development in other organs such as lungs and bone marrow and may have other possible disease associations (Raines et al. (1990) Experimental Pharmacology, Peptide Growth Factors and Their Receptors, Sporn & Roberts, eds., pp. 173-262, Springer, Heidelberg).
One study has examined the effect of inhibition of PDGF B-chain in renal disease: Johnson et al., using a neutralizing polyclonal antibody to PDGF, were able to reduce mesangial cell proliferation and matrix accumulation in experimental mesangioproliferative glomerulonephritis (Johnson et al. (1992) J. Exp. Med. 175:1413-1416). In this model, injection of an anti-mesangial cell antibody (anti-Thy 1.1) into rats resulted in complement-dependent lysis of the mesangial cells, followed by an overshooting reparative phase that resembled human mesangioproliferative nephritis (Floege et al. (1993) Kidney Int. Suppl. 39:S47-54). Limitations of the study of Johnson et al. (Johnson et al. (1992) J. Exp. Med. 175:1413-1416) included the necessity to administer large amounts of heterologous IgG and a limitation of the study duration to 4 days due to concerns that the heterologous IgG might elicit an immune reaction.
Inhibition of PDGF
Specific inhibition of growth factors, such as PDGF, has become a major goal in experimental and clinical medicine. However, this approach is usually hampered by the lack of specific pharmacological antagonists. Available alternative approaches are also limited, since neutralizing antibodies often show a low efficacy in vivo and are usually immunogenic, and given that in vivo gene therapy for these purposes is still in its infancy. Currently, antibodies to PDGF (Johnsson et al. (1985) Proc. Natl. Acad. Sci. U.S.A. 82:1721-1725; Ferns et al. (1991) Science 253:1129-1132; Herren et al. (1993) Biochimica et Biophysica Acta 1173:294-302; Rutherford et al. (1997) Atherosclerosis 130:45-51) and the soluble PDGF receptors (Herren et al. (1993) Biochimica et Biophysica Acta 1173:294-302; Duan et al. (1991) J. Biol. Chem. 266:413-418; Teisman et al. (1993) J. Biol. Chem. 268:9621-9628) are the most potent and specific antagonists of PDGF. Neutralizing antibodies to PDGF have been shown to revert the SSV-transformed phenotype (Johnsson et al. (1985) Proc. Natl. Acad. Sci. U.S.A. 82:1721-1725) and to inhibit the development of neointimal lesions following arterial injury (Ferns et al. (1991) Science 253:1129-1132). Other inhibitors of PDGF such as suramin (Williams et al. (1984) J. Biol. Chem. 259:287-5294; Betsholtz et al. (1984) Cell 39:447-457), neomycin (Vassbotn et al. (1992) J. Biol. Chem. 267:15635-15641) and peptides derived from the PDGF amino acid sequence (Engstrom et al. (1992) J. Biol. Chem. 267:16581-16587) have been reported, however, they are either too toxic or lack sufficient specificity or potency to be good drug candidates. Other types of antagonists of possible clinical utility are molecules that selectively inhibit the PDGF receptor tyrosine kinase (Buchdunger et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92:2558-2562; Kovalenko et al. (1994) Cancer Res. 54:6106-6114).